The Nissan Path

Hi everyone just joined today. Have a late model Pathy, Dec '11, turbo diesel. Tows the caravan really well, but has trouble keeping the batteries charged up in the caravan.
Using arc welding cables thru the vehicle to an anderson plug 50A, then 13mm cable thru the caravan to the batteries.
Simple paralleling of batteries used via ignition relay.
When I turn the fridge onto 12Volt ie, 170Watt heater element, approx 14 amp draw, the batteries deep cycle 200 amp, can no longer charge up.
Noticed on my temp volt meter plugged into drivers cig lighter socket that the alternator puts out 14.6 volts on initial start up and after about 20 mins that has diminished to about 12.6 volts, with or without the caravan connected.
Other forums indicate the ECM system controlling the alternator shuts it down to save fuel and limit pollution.
Has anyone found a way of getting better use out of this 130 amp alternator.
Deep cycle batteries need a constant 14.4 volts to successfully charge them up and this usually takes 1 - 2 hours at least to accomplish not the 20 mins Nissan provides for.

Mike.